A group of four Shia Muslim clerics in Pakistan has been killed in what officials believe was a sectarian attack.

The deaths occurred in the city of Dera Ismail Khan in Pakistan's North West Frontier Province, but there are few details.

The incident coincided with the observance of the Muslim holy month of Muharram during which many Shi'ite Muslims mourn the death of the Prophet Mohammed's grandson, Imam Hussein; security forces have been on high alert to try to prevent violence between followers of the the Sunni and Shia sects.